Mesh type filamentary thermionic cathode emitter and tube using same

ABSTRACT

First and second sets of thermionic cathode emitter filaments are wound into a composite cross-wound helical cathode emitter structure. The helical cathode emitting structure is supported at opposite ends by means of electrical terminals for passing electrical current through the cross-wound filaments. At one end of the composite mesh, the filaments have inwardly directed terminal portions terminating on the terminal support structure, whereby the inwardly directed terminal portions of the filaments permit a certain degree of flexibility to accommodate unequal thermal expansion of the support and mesh and whereby the cooling effect of the terminal structure is generally limited to the inwardly directed terminal portions of the filaments. The terminal structure also includes flexible support fingers to further accommodate unequal thermal expansion.

United States Patent McNees et al.

[111 3,824,424 [451 July 16, 1974 MESH TYPE FILAMENTARY THERMIONIC CATHODE EMITTER AND TUBE USING SAME [75] Inventors: Sterling G. McNees, Los Altos Hills;

James P. Polese, Menlo Park, both 21 App]. No.: 345,052

[52] US. Cl 313/341, 313/272, 313/274,

Polese 313/341 X Primary Examiner.lames W. Lawrence Assistant Examiner-Saxfield Chatmon, Jr.

Attorney, Agent, or FirmStanley Z. Cole; Leon F. Herbert [5 7] ABSTRACT First and second sets of thermionic cathode emitter filaments are wound into a composite cross-wound helical cathode emitter structure. The helical cathode emitting structure is supported at opposite ends by means of electrical terminals for passing electrical current through the cross-wound filaments. At one end of the composite mesh, the filaments have inwardly directed terminal portions terminating on the terminal support structure, whereby the inwardly ,directed terminal portions of the filaments permit a certain degree 'of flexibility to accommodate unequal thermal expansion of the support and mesh and whereby the cooling effect of the terminal structure is generally limited to the inwardly directed terminal portions of the filaments. The terminal structure also includes flexible support fingers to further accommodate unequal thermal expansion.

6 Claims, 4 Drawing Figures PATENTEU JUL 1 s 1914 MESH TYPE FILAMENTARY THERMIONIC CATHODE EMITTER AND TUBE USING SAME BACKGROUND OF THE INVENTION The invention relates in general to mesh type filamentary thermionic cathode emitters and more particularly to cross-wound helical filamentary cathode emitting structure and tubes using same.

DESCRIPTION OF THE PRIOR ART Heretofore, mesh type filamentary thermionic cathode emitters have been formed by cross winding first and second sets of thermionic cathode emitter filaments. The filaments have been terminated at opposite ends in terminal support structuresfor supporting the mesh type cathodeemitter and for passing electrical current through the cross-wound filaments to directly heat the thermionic cathode to emitting temperature. The terminal support structures included cylindrical portions with the ends of the filaments being spot welded to the outer surfaces of the cylindrical terminal structure. Such a mesh cathode and support for same is disclosed and claimed in copending US Application Ser. No. 777,747 filed Nov. 21, 1968 and assigned to the same assignee as the present invention.

The problem with this type of terminal support for the cross-wound helical thermionic cathode emitting structure is that the relatively massive cylindrical terminals provide an end cooling effect for the composite cathode emitter mesh structure. More particularly, the relatively massive terminals operate at a lower temperature than the directly heated filamentary emitters. As a consequence, the terminal portions of the filaments, where connected to the cylindrical portions of the terminal structure, lose heat to the terminals. Thus, the effective emissivelength of the composite mesh filamentary emitter is reduced compared to the actual physical axial length of the composite cylindrical mesh emitter. In high frequency applications, this loss of effective length increases the inductance of the cathode by an undesired amount because the physical length of the cylindrical mesh is longer than desired.

In addition, the relatively massive cylindrical terminal portions provide an interelectrode capacitance at the ends of the cathode emitting structure which is also undesired for high frequency operations. Furthermore, unequal thermal expansion of the cathode terminal support structure relative to that of the composite mesh filament results in undesired distortion of the mesh filament, thereby changing the interelectrode spacing and deleteriously affecting performance and operating life of the tubes using the emitter.

SUMMARY OF THE PRESENT INVENTION The principal object of the present-invention is the provision of an improved mesh type filamentary thermionic cathode emitter and tubes utilizing same.

In one feature of the present invention, the terminal portions of the filament of the composite mesh cathode emitter at one end are bent inwajrdly of the composite emitter and terminated on a central support structure, whereby an axially yieldable support for the composite emitting mesh structure is obtained by the yieldable terminal portions of the filaments to accommodate differences in expansion between the emitter structure and its support, and whereby the end cooling effect of the composite cathode emitting structure is confined at this one end to the inwardly directed portions of the filaments to decrease the inductance and the interelectrode capacitance of the composite cathode emitter.

In another feature of the present invention, the terminal support structure for supporting at least one end of the mesh filamentary emitter, includes a ring portion to which the ends of the filaments are bonded and a radially yieldable support means, such as L-shaped fingers interconnect the ring portion and a central support to allow for differences in thermal expansion of the mesh cathode and its support structure.

Other features and advantages of the present invention will become apparent upon a perusal of the following specification taken in connection with the accompanying drawings wherein:'

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a longitudinal sectional view of a high frequency coaxial electron tube incorporating features of the present invention,

FIG. 2 is a view of the structure of FIG. 1 taken along line 22 in the direction of the arrows,

FIG. 3 is a sectional view of the structure of FIG. 2 taken along line 3-3 in. the direction of the arrows, and

FIG. 4 is a view of the structure of FIG. 2 taken along line 4-4 in the direction of the arrows.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring now to FIG. 1 there is shown an electron tube having four coaxially mounted, cylindrical electrodes consisting of a directly heated mesh type filamentary cathode emitter 10, a control grid 11, a screen grid 12, and an anode 13. The cathode l0 and grids 11 and 12 are mutually aligned and supported by means of supporting structure comprising a cylindrical, copper stem 16 to which a center rod 17 is coaxially joined. The upper end of the cylindrical mesh cathode 10 is attached to the center rod 17 by means of a molybdenum terminal support ring structure 18. Ring structure 18 includes-a radially directed lip portion to which the inner ends of the filaments of the mesh cathode emitter 10 are bonded in a manner more fully disclosed below with regard to FIGS. 2-4. The terminal ring 18 also includes a plurality of L-shaped fingers, each finger including an axially directed portion and a radially inwardly directed portion. The inwardly directed portions of the fingers are affixed as by riveting to a disc affixed to the center rod support 17.

The lower end of the mesh cathode 10 is attached to a molybdenum cup 19 which is brazed to a hollow copper support cylinder 20 which in turn is locked to stem 16 by means of two ceramic annuli 22 and 24. A flexible copper collar 26 is affixed at the lower end of the support cylinder. The collar extends in sandwich fashion between two of three ceramic rings 28 through the tube envelope to provide an externally accessible cathode terminal. Ceramic rings 28, stem 16 and external anode 13 each form portions of the tube envelope as do ceramic cylinder 30 and metallic ring 32. An appropriate jacket, not shown, may be placed about the anode on mounting flange 34 for vapor or water cooling of the anode.

Referring now to FIGS. 2-4, there is shown the improved mesh type filamentary cathode emitter 10 of the present invention. The filamentary emitter 10 includes two sets of generally parallel helical thoriated tungsten filamentary emitting wires 35 and 36. The two sets of filamentary emitters 35 and 36 are wound in a cross-wound helical mesh structure and the crossover points are fused as by spot welding in a certain predetermined pattern (See FIG. 4) to define a relatively rigid cylindrical mesh type filamentary cathode emitter structure 10.

At the upper end of the crosswound helical mesh cathode 10, the filaments 35 and 36 are bent inwardly of the outer cylindrical surface of the mesh 10 and terminated on 'the terminal support ring 18, as by spot welding and subsequent RF brazing. The inwardly directed terminal portions of the filaments 35 and 36 are axially defiectable as indicated by the vertical arrow 37 of F IG. 3 to relieve stress that may otherwise be transmitted from the terminal support ring 18 to the mesh cathode structure 10.

This relative vertical deflection between the mesh cathode l and its support structure 18 reduces distortion of the cylindrical outer surface of the mesh cathode thereby preserving the right circular cylindrical shape of the mesh cathode emitter 10 and the precise interele'ctrode dimensions between the emitter 10 and the adjacentgrid ll.

In addition, the cooling effect of the relatively massive' terminal support ring 18 on the adjacent ends of the filamentary cathode emitters 35 and 36 is generally limited to the inwardly directed portions of the emitter filaments such that the effective axial length of the composite cathode emitter 10, at the end thereof adjacent the support ring 18, is essentially the full physical axial length of the composite emitter structure 10 at this end. In this manner, the inductance and interelectrode capacitance of the upper terminal structure of the-mesh cathode emitter l0 is minimized as contrasted with the prior art cathode utilizing a cylindrical upper support structure having an outer diameter equal to the outer diameter of the mesh cathode l0.

In addition, the L-shaped support fingerportions 38,

t of the upper terminal ring structure l8, allow relative radial deflection of the support ring 18 relative to the rigid central support structure 17, as indicated by the arrow 39 of FIG. 3. This further reduces theradial stresses coupled to the mesh cathode 10 from the cathode support structures 17 and 18. In a typical example, the upper terminal ring 18 has a thickness as of 0.040 inches and the helical filaments 35 and 36 preferably make an angle of between 20 and with the plane normal to the longitudinal axis of the composite cylindrical mesh cathode 10.

At the lower or other end of the composite mesh cathode 10, the filaments and 36 are spot welded and brazed to the outer cylindrical surface of the lower terminal support cup 19. The axial length of the cylindrical portion of the cup 19 to which the filaments 35 and 36 are bonded is preferably kept to a minimum to reduce the interelectrode capacitance and the inductance of the cathode structure.

It would be desirable to turn inwardly the filaments 35 and 36 at the lower end of the composite structure in the same manner as the filaments are terminated at the upper end. However. the cross-wound helical composite mesh emitter 10 is formed by winding the filaments 35 and 36 on a mandrel and bonding the filaments to the support structures 18 and 19 while the wires are wound on the mandrel. Thus, the relatively large opening at the lower end of the composite mesh filament is required for removal of the mandrel.

What is claimed is:

1. in a mesh type filamentary cathode emitter:

first and second sets of thermionic cathode emitter filaments wound into a composite crosswound helical cathode emitter structure;

terminal support means for supporting and making electrical connection to opposite ends of said first and second sets of filaments for passing electrical current through said cross-wound filaments to heat said filaments to thermionic cathode emission temperature; and

said filaments of' said first and second sets, at a first end of said composite cross-wound helical structure, having continuing portions inwardly directed of said cross-wound structure toward said terminal support means, said inwardly directed continuing portions being secured to said terminal support means, and the length of said inwardly directed continuing portions being such that said first end of the cross-wound helical structure is spaced outwardly from said terminal support means, whereby said inwardly directed continuing portions of said first and second sets of filaments are relatively free to flex in the axial direction of said composite cross-wound helical structure and whereby the cooling effect of said terminal support means at said first end of said helical structure is generally limited to said inwardly directed continuing portions of said filaments.

2. The apparatus of claim 1 wherein said first and second sets of filaments are made oftungsten wire.

3. The apparatus of claim 1 wherein said first and second cross-wound filaments are fused together at crossover points thereof.

4. The apparatus of .claim'l wherein said helical filamentsmake an angle of between 20 and 30 with the plane perpendicular to the longitudinal axis of said cross-wound helical structure.

5. The apparatus of claim 1 wherein said terminal support means at said first end of said composite crosswound helical structure comprises a ring portion to which the ends of said filaments are bonded, and radially yieldable support means connected to said ring portion and extending inwardly of said ring portion for supporting said ring portion from a central support structure.

6. In an electron tube:

thermionic cathode emitter means supply of electrons;

said emitter means comprising, first and second sets of thermionic cathode emitter filaments wound into a composite cross-wound helical meshcathode emitter structure; terminal support means for supporting and making electrical connection to opposite ends of said first and second sets of filaments for passing electrical current through said cross-wound filaments to heat said filaments to thermionic cathode emission temperature; I

said filaments of said first and second sets, at a first end of said composite cross-wound helical structure, having continuing portions inwardly directed of said cross-wound structure toward said terminal support means, said inwardly directed portions of for providing a inwardly directed continuing portions of said filaments;

anode means surrounding said composite crosswound cathode emitter; and

control grid means surrounding said cathode emitter and being interposed between said cathode emitter and said anode means for controlling the flow of electrons from said cathode emitter to said anode means. 

1. In a mesh type filamentary cathode emitter: first and second sets of thermionic cathode emitter filaments wound into a composite crosswound helical cathode emitter structure; terminal support means for supporting and making electrical connection to opposite ends of said first and second sets of filaments for passing electrical current through saId crosswound filaments to heat said filaments to thermionic cathode emission temperature; and said filaments of said first and second sets, at a first end of said composite cross-wound helical structure, having continuing portions inwardly directed of said cross-wound structure toward said terminal support means, said inwardly directed continuing portions being secured to said terminal support means, and the length of said inwardly directed continuing portions being such that said first end of the cross-wound helical structure is spaced outwardly from said terminal support means, whereby said inwardly directed continuing portions of said first and second sets of filaments are relatively free to flex in the axial direction of said composite cross-wound helical structure and whereby the cooling effect of said terminal support means at said first end of said helical structure is generally limited to said inwardly directed continuing portions of said filaments.
 2. The apparatus of claim 1 wherein said first and second sets of filaments are made of tungsten wire.
 3. The apparatus of claim 1 wherein said first and second cross-wound filaments are fused together at crossover points thereof.
 4. The apparatus of claim 1 wherein said helical filaments make an angle of between 20* and 30* with the plane perpendicular to the longitudinal axis of said cross-wound helical structure.
 5. The apparatus of claim 1 wherein said terminal support means at said first end of said composite cross-wound helical structure comprises a ring portion to which the ends of said filaments are bonded, and radially yieldable support means connected to said ring portion and extending inwardly of said ring portion for supporting said ring portion from a central support structure.
 6. In an electron tube: thermionic cathode emitter means for providing a supply of electrons; said emitter means comprising, first and second sets of thermionic cathode emitter filaments wound into a composite cross-wound helical mesh cathode emitter structure; terminal support means for supporting and making electrical connection to opposite ends of said first and second sets of filaments for passing electrical current through said cross-wound filaments to heat said filaments to thermionic cathode emission temperature; said filaments of said first and second sets, at a first end of said composite cross-wound helical structure, having continuing portions inwardly directed of said cross-wound structure toward said terminal support means, said inwardly directed portions of said filaments at said first end being secured to said terminal support means, and the length of said inwardly directed continuing portions being such that said first end of the cross-wound helical structure is spaced outwardly from said terminal support means, whereby said inwardly directed continuing portions of said first and second sets of filaments are relatively free to flex in the axial direction of said composite cross-wound helical structure and whereby the cooling effect of said terminal support means at said first end is generally limited to said inwardly directed continuing portions of said filaments; anode means surrounding said composite cross-wound cathode emitter; and control grid means surrounding said cathode emitter and being interposed between said cathode emitter and said anode means for controlling the flow of electrons from said cathode emitter to said anode means. 